The difference is that Brave Bird has recoil damage, where firing off things like Sky Attack have no recourse at all. They hit for 120 power all the time, without the chance to miss. Things like Hydro Pump, Fire Blast, Stone Edge, and Focus Blast can all miss (and tend to all the time =P). Giving moves like that absolutely no recourse (and I'd assume this would mean Hyper Beam and Giga Impact wouldn't have to recharge either) would just be way too strong.